The present invention relates generally to pruning of standing trees. More particularly it relates to a pruning head mounted on the boom of forestry equipment that operates to sever attached limbs from a standing tree as the head moves up the tree.
In modern commercial forestry operations where trees are grown more like crops with scientifically applied periodic attention it has been found that the growing cycle can be shortened with the same amount, more, and/or better quality wood volume being produced per acre of land. For example, such techniques can include preparation of the ground prior to planting new seedlings, replanting with genetically improved seedlings, fertilization, pre-commercial thinning, and pruning of limbs. It is the pruning step to which the present invention is directed. When a tree grows in the early years limbs naturally occur and grow laterally out from the main stem of the tree. As the main growth continues upwardly in the stem and as the tree reaches substantial vertical height, usually the lower limbs will simply die and will either fall by natural causes or will remain with the tree until final harvest. Of course, even in the final stages of a tree's life, there will be limbs at the upper reaches of a standing tree.
It has been learned that a pre-commercial thinning step where the smaller, weaker trees are thinned from a stand will enhance the growth of the remaining trees. This pre-commercial thinning step can take place approximately ten years after the initial replanting. Another beneficial step that has been learned is a pruning step after a period from approximately 10-14 years after the pre-commercial thinning step. If the remaining trees are pruned to a height of approximately 20 feet above the ground, a substantial amount of improved wood will be obtained after final harvest which will then usually occur after the passage of another 10-15 years. What has been learned is that if the limbs up to the approximate height of twenty feet are pruned away, diameter growth will continue to add layers of clear wood over the length where limbs have been pruned away. This has a direct relationship to increasing the value of the wood by removing the limbs that would become knots in finished lumber products.
Such knots tend to chip or drop out when the timber is being finished and also spoil the finish for many types of products. In plywood, knots may drop out to cause imperfections in the surface finish so a veneer of clear wood is often sought for the surface layer. In addition, the presence of knots prevents the even distribution of stress along a section of timber. If the limbs or branches are pruned close to the trunk, wood which grows beyond the stubs of the branches as the tree grows in diameter is knot free. The closer the branch is pruned to the trunk, and consequently the shorter the stub, the greater the yield of knot free timber.
Others have contemplated tree pruning and there are several examples in the prior art of tree pruning mechanisms that are designed to be motivated up a certain length of a standing tree to remove limbs and branches. One such device is that disclosed in U.S. Pat. No. 3,385,332 to Otterbach et al in which a motorized device travels up the tree stem in a helical path and has circular saws or chain saws that are operable to sever the limbs and branches. Another device is that disclosed in the Annual Report (1976-1977) of CSIRO Division of Forest Research in Australia wherein a prototype mechanized high pruning device consists of a pruning head bearing an array of six knives which is attached to a vertical mast. The mast is carried on a pivotal mounting and aligned with the tree trunk by means of a pair of hydraulic cylinders. The knives in the head are kept in close contact with the trunk and the branches are pruned as the head is moved up the mast.
It is also known that a knife chain may be used after it is wrapped about a stem for delimbing the stem when the chain or the stem is moved relative to the other. An example of such a delimbing arrangement is disclosed in U.S. Pat. No. 3,643,708 to Lindblom. The Lindblom patent discloses a pair of pivotally mounted curved arms on which are mounted linked together knife blades which serve to shear the limbs. Another similar disclosure is contained in the German Offenlegungsschraft No. 2,720,149.
All of the above-cited prior art disclosures have deficiencies with respect to providing a cost effective, quality tree pruner. What is needed is a tree pruning mechanism that can be manipulated quickly in the forest to maneuver into position to prune selected trees. Once in position, the pruning mechanism should then act quickly to remove those limbs and branches that are desirable for removal close to the bole without excess damage to the tree. In addition, of course, unless a robotic version is developed, safety is essential for the operator who is in close proximity to the actual pruning. Obviously when limbs and branches are pruned from a standing tree, the material must be safely handled.
Accordingly from the foregoing one object of the present invention is to quickly and efficiently prune selected trees to a predetermined height.
Another object of this invention is to provide a pruning mechanism that operates to shear the limbs and branches from the tree very close to the surface, thereby leaving very short stubs.
Yet a further object is to provide a pruning mechanism that will not significantly damage the tree during the pruning operation.
Still a further object of the present invention is the provision of a multiple station pruning mechanism having a plurality of shearing knives which will serve to prune the tree faster than a single shearing station mechanism.
These and many additional objects of the present invention will be better understood and more fully appreciated upon reading the specification to follow in conjunction with the attached drawings.